Pulmonary surfactant is complex of lipids and 4 specific proteins, SP-A, SP-B, SP-C, and SP-D, that functions to reduce surface tension at the air- liquid interface. A deficiency of surfactant, as is seen in the Infant Respiratory Distress Syndrome of the premature newborn, results in difficulty in breathing and inadequate oxygenation. In addition to lowering surface tension in the alveoli, recent studies suggest that the surfactant proteins SP-A and SP-D may play a role in pulmonary host defense against infection. It is well established that the alveolar type II cell synthesizes and secretes surfactant. However, the factors that regulate clearance of surfactant from the airspaces are much less well understood. The long term goal of this study is to investigate the factors that regulate surfactant clearance and metabolism and specifically to test the hypotheses that both type II cells and macrophages play important roles in surfactant clearance and that surfactant protein SP-A is a key regulator of clearance and turnover. During the previous funding period, it was demonstrated that SP- A binds to receptors and is internalized by both type II cells and macrophages. The fate of SP-A and the lipid internalized by the two cell types differs greatly. Macrophages rapidly degrade both surfactant lipid and SP-A. In contrast, much of the lipid internalized by type II cells in the presence of SP-A is "recycled", that is, internalized and eventually resecreted. The factors that target SP-A to type II cells or macrophages are not known but may include interaction of SP-A with different receptors, interaction of different domains of SP-A with each specific cell type, or interaction of SP-A with other surfactant components, such as lipids or other surfactant proteins, including SP-D, which may mask or expose domains of SP-A. We propose to characterize further the mechanism of surfactant clearance by investigating five specific aims. Aim #1 is to map the domains of SP-A that mediate its interaction with type II cells and macrophages. Aim #2 is to isolate and characterize the type II cell SP-A receptor. Aim #3 is to characterize further the metabolism of SP-A by type II cells and macrophages. Aim#4 is to characterize the effects of SP-A on clearance of phosphatidylglycerol and cholesterol. Aim #5 is to define the clearance and metabolism of SP-D and to investigate the role of SP-D in regulation of surfactant metabolism. If successful, these studies will provide new information regarding the regulation of clearance and metabolism of surfactant which could lead to more rationale design of surfactant therapies. An understanding of the factors that regulate interaction of surfactant proteins with alveolar cells could have important implications in design of new SP-A peptide based therapies. with the goal of targeting SP-A toward regulation of type Il cell mediated functions (surfactant turnover) or macrophage functions (immune responses).